Optical fiber amplifiers and lasers are now well-established as a technology having undergone remarkable progress in their development over the last several years. Early on they were of simple form comprising a gain species dispersed in a core surrounded by a cladding. The core, which serves as the host for the gain species, was usually of silica glass, but matrix material and hosts of fluoride or phosphate based glasses have been reported. The concentration of the gain species, the length of fiber, and the pump power are arranged so that losses are overcome by gains to provide amplification or laser action, where the fiber has been provided with suitable feedback. Output powers initially were on the order of milliwatts.
Initially, pumping was via the side of the core with flashtubes, but this was replaced by end pumping the core (i.e., "end-fired"). However, end pumping has its problems because it is not very efficient, even with laser diodes, so output power was limited to a rather low milliwatt level because all of the available pump power could not be fully utilized due to the practically realizable physical and optical properties of sources and fibers. With the advent of laser diode arrays operating at wavelengths suitable for pumping a number of rare-earth ions, it appeared that such sources were natural candidates for achieving higher output powers. But, because of their beam quality, coupled with inherent practical limits on optical fiber numerical apertures (i.e., solid acceptance cones), it was still physically impossible to efficiently couple these higher power sources into cores, especially single-mode cores which would require a single-mode pump for efficient coupling.
However, Snitzer et al. disclosed an elegant solution to this problem in U.S. Pat. No. 4,815,079, and provided a significant improvement over an earlier approach by Maurer, as described in his U.S. Pat. No. 3,808,549. In the Snitzer et al. scheme, now referred to as "cladding pumping", a single-mode core containing the active ion is surrounded by an undoped inner multimode cladding of lower index than that of the core and is of a special geometry for efficient pumping. This, in turn, is surrounded by an outer cladding of yet lower index of refraction. Pump light is launched into the inner cladding and is confined by total internal reflection at the interface between claddings to propagate down the inner cladding, which is a core-like structure with respect to the outer cladding. The inner cladding, being multimode, is obviously physically larger than the core and therefore presents a better target, and the numerical aperture, being a function of the indices of the inner and outer claddings, is made as large as possible to more efficiently receive pump power. As pump power propagates down the inner cladding, it is progressively absorbed by the core to provide the population inversion necessary for gain and subsequent laser action with suitable feedback. This scheme is a hybrid having the character of both longitudinal and transverse pumping, and has the great merit of efficiently coupling available pump power from an incoherent source to a single-mode core to provide single-mode output. Inner cladding geometries that have been found efficacious include elongated slab configurations, like the rectangle, and a configuration in which a core is eccentrically located inside of the inner cladding.
Even though the Snitzer et al. configurations represent significant methods for enhanced coupling for pump power provide a single mode output in fiber format, it is a primary object of the present invention to provide a variety of cladding shapes for use in efficient cladding pumping of fiber amplifiers and lasers.
It is a further object of the present invention to provide a variety of efficient cladding shapes for use with a variety of pump radiation distribution patterns.
It is another object of the present invention to provide efficient cladding shapes to match available pump characteristics.
It is another object of the present invention to provide an optical fiber with optimal radiation coupling efficiency.
It is another object of the invention to provide such an optical fiber in which the fiber core is single-mode.
It is another object of the invention to provide such an optical fiber which will provide an even distribution of radiation modes within the fiber inner cladding.
It is yet another object of the invention to provide an optical fiber in which the fiber radiation coupling efficiency is not a function of the location of the fiber core.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the following detailed description is read in connection with the drawings.